Solid state photonic devices are a class of devices in which the quantum of light, the photon, plays a role. Photonic devices are often classified into three categories: light sources (light-emitting diodes, lasers, diode lasers etc.), photodetectors (photoconductors, photodiodes etc.) and energy conversion devices (photovoltaic cells) [S. M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981)]. All three are important. Because photonic devices are utilized in a wide range of applications, they continue to provide a focus for research laboratories all over the world.
The solid state photonic devices function by utilizing electro-optical and/or opto-electronic effects in solid state materials. Because the interband optical transition (in absorption and/or in emission) is involved in photonic phenomena and because photon energies from near infrared (IR) to near ultraviolet (UV) are of interest, the relevant materials are semiconductors with band gaps in the range from about 1 to about 3 eV. Typical inorganic semiconductors used for photonic devices include Si, Ge, GaAs, GaP, GaN, SiC, (In,Ga)N, and the like. The Group III-nitrides, such as GaN, and (In,Ga)N have been widely studied.
In the (In,Ga)N alloy semiconductor system, the band gap can be varied from 1.9 to 3.4 eV. As a result, (In,Ga)N alloys are useful for the fabrication of optoelectronic devices operating in the wavelength range from the red to ultraviolet. Furthermore, Group III-nitrides generally are distinguished by their high thermal conductivity and physical hardness, making them suitable for high temperature applications. Group III-nitride p-n junction light-emitting diodes (LEDs) grown by metal organic chemical vapor deposition (MOCVD) emit in the blue-green spectral region.
Optical characterization of GaN has revealed that, in addition to the emission originating from the interband transition (380 nm), there is also a strong defect-mediated yellow emission around (550 nm). In general, the latter is more intense than the former; hence GaN itself is not believed to be appropriate as the emitting layer in devices. [W. Grieshaber, E. F. Schubert, I. D. Goepfert, R. F. Karlicek, Jr., M. J. Schurman, and C. Tran, J. Appl. Phys. 80, 4615 (1996).] On the other hand, by using a mixed Group III metal nitride, InGaN, as the emitting layer in an LED, the emission from the interband transition can be made to be dominant, and the emission wavelength can be tuned by varying the mole fraction of In. [S. Nakamura, M. Senoh, M. Iwasa, S. Nagahama, T. Yamada, and T. Mukai, Jpn. J. Appl. Phys. 34, L1332 (1995); H. Morko.cedilla. and S. N. Mohammad, Science 267, 51 (1995).] A number of schemes have been implemented for Group III-nitride-based LEDs, including the use of InGaN quantum wells and Zn-doped InGaN
Although different colors are available from devices using InGaN as the active layer, InGaN films have proven to be difficult to grow as a result of the high volatility of indium at normal nitride growth temperatures. This makes the achievement of white light, comprised of red, green, and blue spectral regions, and/or multiple color LEDs from the InGaN system difficult and costly.
Inorganic semiconductor LEDs have recently achieved sufficiently high efficiencies that they have become competitive with other lighting technologies. Until 1995, bright blue LEDs were the missing color which prevented full-color LED based displays. Now, using Group III-nitride technology, bright blue LEDs with luminous efficiency of 6 lumens/W (7% quantum efficiency) and bright green Group III-nitride-based LEDs with luminous efficiencies over 30 lm/W (5% quantum efficiency) have been realized. The evolution over time of this LED technology is illustrated in FIG. 1.
Unfortunately, however, Group III-nitride technology has not been able to obtain bright red and yellow emission because of a serious drop in luminous efficiency at these longer wavelengths. This is shown in FIG. 2. Therefore GaN chips must be combined with AlGaInP chips to achieve white and full-color products. This multiple chip solution is costly and time consuming; a monolithic source of white light would be much more desirable.
As the quantum efficiency of the Group III-nitride LEDs is further improved from the present level of 7% to anticipated levels in excess of 15%, major opportunities will become possible; in particular, white lighting sources which are substantially more efficient than tungsten light bulbs can be anticipated.
One method of obtaining a monolithic source of white light is to combine Group III-nitride LEDs with phosphors. This approach uses the Group III-nitride source to pump fluorescent films such as ZnS and ZnCdS, as described in Y. Sato, N. Takahashi, and S. Sato, Jpn. J. Appl. Physics 35, L838 (1996). This method produces white light by using red, green and blue phosphors. A disadvantage of this technique is that the phosphors reabsorb higher energy photons, thereby reducing the net efficiency of the light emission. In addition, phosphors degrade under high drive currents and have broad emission spectra which reduce the color purity.
Conjugated polymers are a novel class of semiconductors which combine the optical and electronic properties of semiconductors with the processing advantages and mechanical properties of polymers. Semiconducting polymers typically have band gaps in the range from 1 to 3 eV. The molecular structures of a few important examples of semiconducting polymers are shown in FIG. 3. Because of the sp.sup.2 p.sub.z bonding of these planar conjugated macromolecules, each carbon is covalently bonded to three nearest neighbors (two carbons and a hydrogen); and there is formally one unpaired electron per carbon. Thus, the electronic structure (semiconductor or metal) depends on the number of atoms per repeat unit. For example the repeat unit of poly(paraphenylene vinylene), ("PPV"), contains eight carbons; PPV is a semiconductor in which the fundamental p.sub.z -band is split into eight sub-bands. The energy gap of the semiconductor, the .pi.-.pi.* gap, is the energy between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.
When functionalized with flexible side chains, for example, poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene), ("MEH-PPV"), see FIG. 3, these materials become soluble in common organic solvents and can be processed from solution at room temperature into uniform, large area, optical quality thin films [D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991)]. Because of the large elongation to break which is a characteristic feature of polymers, such films are flexible and easily fabricated into desired shapes that are useful in novel devices [G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature 357, 477 (1992).]
Many conjugated polymers exhibit relatively high photoluminescence (PL) efficiencies with emission that is shifted sufficiently far from the absorption edge that self-absorption is minimal. The absence of self-absorption by the polymer film is a critical advantage of this invention. For example, when inorganic phosphors pumped by Group III-nitride LEDs are used with phosphors to create white light, the self absorption of the phosphor limits the overall efficiency and complicates the achievement of specific desired colors.
Light-emitting diodes have also been fabricated using semiconducting, luminescent polymers as the active materials. Thin film devices in the sandwich (multi-layer thin film) configuration comprise an active luminescent, semiconducting material laminated between two planar electrodes. One of the electrodes is made semitransparent, thereby allowing the emitting light to exit from the device. Inorganic materials (such as ZnS:Mn), organic materials such as organic dye molecules [C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett. 51, 913 (1987); C. W. Tang, S. A. Van Slyke, and C. H. Chen, J. Appl. Phys. 65, 3610 (1989)] and conjugated polymers [J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature 347, 539 (1990); D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991)] have been used in this general type of electroluminescent device.
Although polymer LEDs are promising for applications in emissive displays, they suffer many serious disadvantages compared with inorganic LEDs such as those fabricated from the Group III-nitrides. In particular, the efficiencies are relatively low; quantum efficiencies are below 3% (photons per electron) and often below 1%. Moreover, the achievement of long operating life and long shelf life for polymer LEDs remains a difficult and unresolved problem. Finally, the mechanism of operation of the polymer LED requires that carrier injection be optimized and balanced by matching the electrodes to the electronic structure of the semiconducting polymer. For optimum injection, the work function of the anode should lie at approximately the top of the valance band, E.sub.v, (the .pi.-band or highest occupied molecular orbital, HOMO) and the work function of the cathode should lie at approximately the bottom of the conduction band, E.sub.c, (the .pi.*-band or lowest unoccupied molecular orbital, LUMO). Thus, for each new polymer with a different band-gap and therefore a different emission color, a different pair of electrode materials must be developed, a tedious and difficult task.
Semiconducting, luminescent polymers are also potentially interesting as laser materials. In semiconducting polymers, the emission is at longer wavelengths than the onset of significant absorption (the Stokes shift). Because of the spectral Stokes shift between the absorption and the emission, there is minimal self-absorption of the emitted radiation [F. Hide, M. A. Diaz-Garcia, B. J. Schwartz, M. R. Andersson, Q. Pei, and A. J. Heeger, Science 273, 1833 (1996)]. Thus, in semiconducting luminescent polymers, self-absorption does not make the materials lossy. Moreover, since the absorption and emission are spectrally separated, pumping the excited state via the .pi. to .pi.* transition does not stimulate emission. Thus, by pumping the .pi. to .pi.* transition, one can achieve an inverted population at relatively low pump power.
Optically pumped laser emission has been reported from MEH-PPV in dilute solution in an appropriate solvent, in direct analogy with conventional dye lasers [D. Moses, Appl. Phys. Lett. 60, 3215 (1992); U.S. Pat. No. 5,237,582]. In this application, the diluted and dissolved luminescent polymer serves as the laser dye. More recently, semiconducting polymers in the form of neat undiluted films have been demonstrated as being useful as the active luminescent materials in solid state lasers [F. Hide, M. A. Diaz-Garcia, B. J. Schwartz, M. R. Andersson, Q. Pei, and A. J. Heeger, Science 273, 1833 (1996); N. Tessler, G. J. Denton, and R. H. Friend, Nature 382, 695 (1996)] In addition, laser action has been achieved in dilute films and solutions of semiconducting polymers containing a dispersion of titanium dioxide (TiO.sub.2) nanoparticles. In this case, the nanoparticles serve to multiply scatter the emitted light such that the path length of the emitted photon exceeds the gain length above a certain excitation threshold, and an external resonant cavity is not used. [F. Hide, B. J. Schwartz, M. A. Diaz-Garcia, and A. J. Heeger, Chem. Phys. Lett. 256, 424 (1996).] Thus, when pumped by blue emitting sources of sufficient intensity (for example by Group III-nitride or ZnSe based LEDs or Group III-nitride or ZnSe based laser diodes), electrically pumped laser emission can be realized for colors spanning the entire visible spectrum.
Construction of solid state lasers using semiconducting (conjugated) polymers requires that two criteria be fulfilled: First, the active polymer medium must exhibit stimulated emission (SE) when excited optically or electrically; second, some type of resonant structure must enable the emitted photons to travel a distance greater than the gain length in the excited polymer. The requirement that the emitted photons must travel distances longer than the gain length in the excited medium, can be achieved by using resonant structures; for example, cavities or waveguides. Diverse methods, including external mirrors, distributed feedback (DFB), dielectric mismatch reflection, and microcavities are commonly used. The use of semiconducting polymers as materials for solid state lasers has been disclosed in detail in U.S. Patent Application 60/022,164, [titled Conjugated Polymers as Materials for Solid State Lasers]; incorporated herein by reference.